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Riboflavin Metabolism – Vitamin B2 in the Body

Riboflavin metabolism describes how the body absorbs, activates, and uses riboflavin (vitamin B2) – an essential process for energy production and numerous biochemical reactions.

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Things worth knowing about "Riboflavin Metabolism"

Riboflavin metabolism describes how the body absorbs, activates, and uses riboflavin (vitamin B2) – an essential process for energy production and numerous biochemical reactions.

What Is Riboflavin Metabolism?

Riboflavin metabolism encompasses all biochemical processes involved in the absorption, activation, and utilization of riboflavin, commonly known as vitamin B2. Riboflavin is a water-soluble vitamin that the human body cannot synthesize on its own and must therefore obtain through diet. In the body, it is converted into active coenzymes that are essential for numerous vital reactions.

Absorption and Transport

Riboflavin is actively absorbed in the small intestine via specialized transport proteins, primarily RFVT1, RFVT2, and RFVT3 (Riboflavin Transporters 1–3). In the intestinal mucosa, liver, heart, and other organs, riboflavin is rapidly converted into its biologically active forms. In the bloodstream, it is mainly transported bound to albumin and other proteins.

Activation: Formation of the Coenzymes FMN and FAD

Riboflavin exerts its biological effects only after conversion into two key coenzymes:

FMN (Flavin Mononucleotide): Formed by phosphorylation of riboflavin through the enzyme riboflavin kinase.
FAD (Flavin Adenine Dinucleotide): Synthesized from FMN by FAD synthetase; FAD is the more widely used of the two coenzymes.

Both coenzymes act as electron carriers in numerous enzymatic reactions and are therefore referred to as flavocoenzymes. The enzymes they work with are called flavoproteins.

Functions in Metabolism

FMN and FAD are involved in a wide range of metabolic processes:

Energy metabolism: FAD is essential in the citric acid cycle (as a coenzyme of succinate dehydrogenase) and in the mitochondrial electron transport chain (Complexes I and II).
Fatty acid oxidation: FAD-dependent enzymes are required during the beta-oxidation of fatty acids.
Amino acid metabolism: Flavoenzymes participate in the deamination of amino acids.
Activation of other vitamins: Riboflavin is necessary for the conversion of vitamin B6 and folate into their active forms.
Antioxidant protection: FAD serves as a coenzyme for glutathione reductase, which regenerates the antioxidant glutathione.

Degradation and Excretion

Excess riboflavin is stored only to a limited extent. Surplus riboflavin is primarily excreted via urine, which may appear yellowish-green at higher intake levels. Small amounts are broken down by oxidation and excreted as lumiflavin or lumichrome.

Disorders of Riboflavin Metabolism

Both riboflavin deficiency and rare genetic disorders affecting riboflavin metabolism can impair health:

Riboflavin deficiency (ariboflavinosis): Leads to mucosal inflammation, skin changes, corneal clouding, and impaired energy metabolism.
Riboflavin Transporter Deficiency Syndrome (RTDS): A rare genetic disorder caused by defects in RFVT2 or RFVT3, leading to neuromuscular symptoms such as muscle weakness and hearing loss. High-dose riboflavin supplementation can improve symptoms.
Multiple Acyl-CoA Dehydrogenase Deficiency (MADD): A metabolic disorder affecting FAD-dependent mitochondrial enzymes.

Dietary Sources and Recommended Intake

Since the body cannot synthesize riboflavin, adequate dietary intake is essential. Good sources include milk and dairy products, liver, eggs, fish, whole grains, and green leafy vegetables. The World Health Organization (WHO) recommends a daily intake of approximately 1.1–1.3 mg for adults, with slightly higher amounts during pregnancy and lactation.

References

World Health Organization (WHO): Riboflavin. In: Vitamin and Mineral Requirements in Human Nutrition. 2nd edition. WHO Press, Geneva, 2004.
Bates CJ. Riboflavin. In: Erdman JW, Macdonald IA, Zeisel SH (eds.). Present Knowledge in Nutrition. 10th edition. Wiley-Blackwell, 2012.
Jaeger B, Bosch AM. Clinical presentation and outcome of riboflavin transporter deficiency: mini review after five years of experience. Journal of Inherited Metabolic Disease. 2016;39(4):559–564.

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